The present invention relates to an implantable medical device, such as a pacemaker or ICD (implantable cardioverter defibrillator) which employ electronic components including integrated electronic circuitry within a hermetically sealed housing. More particularly, to the present invention relates to a system for suppressing alpha particles, which may be directed towards the electronic components.
Implantable stimulation devices of the type having electronic circuit components are well known in the medical arts. In one particularly common form, the implantable device comprises a pacemaker unit having an appropriate electrical power supply and related control circuitry for use in electrically stimulating a patient muscle, such as the heart. Such pacemaker units commonly include an hermetically sealed case or housing within which the power supply and control circuitry are protectively encased, in combination with one or more conductive pacemaker leads extending from the housing to the selected muscle structure within the patient. Feed-through terminals on the pacemaker housing accommodate hermetically sealed passage of electrical conductors to the housing exterior for appropriate connection to an implantable lead.
These implantable devices such as pacemakers and cardiac defibrillators employ the implantable electrical leads for passing electrical signals between the device and the heart. To insure an uninterrupted electrical path between the device and the heart, a strong reliable mating between the implantable leads and the connector top of the device is necessary. It is also desirable to seal the entrance of the connector top of the device from body fluids.
Substantial advances have been gained over the years in overcoming problems of sealing and connecting the components, for example. However, another persistent problem addressed by the present invention concerns the presence of alpha particles in the environment which can disrupt the electronic circuitry, specifically, data in Static Random Access Memory (SRAM) integrated circuits which are at the core of an implantable medical device. Alpha particles are naturally occurring and can be emitted by numerous materials that are used in the construction of pacemaker and ICD devices. Pacemakers and ICDs routinely use SRAM circuitry so it is imperative that the SRAM devices be protected from alpha particles.
Alpha particles can disrupt data in Static Random Access Memory (SRAM) integrated circuits. Alpha particles have low energy and can be stopped by a thin layer of material that does not itself emit alpha particles. The semiconductor industry has developed many commercially available materials used to coat semiconductors used in multi-chip modules (MCM). One family of materials is typically used to xe2x80x9cglob topxe2x80x9d integrated circuits. These materials are deposited onto the integrated circuits and then cured to a hard coating. The inherent drawbacks of using glob top materials is that rework and failure analysis are severely curtailed by the hard coating.
Such efforts made previously to shield electronic circuitry against external influences are found in the patent literature. Typical instances of a known techniques are disclosed in U.S. Pat. No. 5,019,409 to Wesling et al. and U.S. Pat. No. 5,334,245 to Hartnett et al., each of which concerns a method of, and apparatus for, dispensing a thin coating of a highly viscous encapsulant liquid on the top surface of a semiconductor device. U.S. Pat. No. 6,239,479 to Hwang et al. discloses a thermal neutron shield for integrated circuits which deters absorption of thermal neutrons by circuit constituents to form unstable isotopes with subsequent decay which generates bursts of charge which may upset stored charge and create soft errors.
It is noteworthy that none of the prior art is concerned with implantable medical devices but is in light of the foregoing that the present invention was conceived and has now been reduced to practice.
The present invention relates to an implantable medical device in the form of a cardiac stimulation device such as a pacemaker or defibrillator which includes a hermetically sealed housing having an inner peripheral surface, an electronic component such as an integrated electronic circuit sensitive to bombardment by alpha particles and located within the housing spaced from the inner peripheral surface. Blocking material is provided intermediate the inner peripheral surface of the housing and the electronic component capable of suppressing alpha particles having an energy level up to about 15 mev directed towards the electronic component. The blocking material has a thickness of at least 0.010 inches; it may be sheet material; it may be congruently attached to the inner peripheral surface of the housing; or it may be a liquid coating applied to the inner peripheral surface and subsequently cured.
Experiments have shown that as little as 0.010 of an inch of certain glob top materials will stop 100% of alpha particles up to 15 mev of energy. By coating the inside surface of the housing of the MCM, for example, the alpha particles can be stopped without coating the integrated circuit directly. This technique protects the SRAM from upset and yet allows for easy rework and failure analysis.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.